Reactor packing with preferential flow catalyst

ABSTRACT

The present invention relates to reactor tubes packed with a catalyst system employed to deliberately bias process gas flow toward the hot tube segment and away from the cold segment in order to reduce the circumferential tube temperature variation.

FIELD OF INVENTION

The present invention relates to a reactor tube packed with a catalystsystem employed to deliberately bias process gas flow toward the tubewall side of above-average incident heat flux, herein called the “highflux side”, and away from the tube wall side of below-average incidentheat flux, herein called the “low flux side”, and as further definedbelow, in order to reduce the circumferential tube temperaturevariation.

BACKGROUND OF THE INVENTION

Steam methane reforming processes are widely used in the industry tomake hydrogen and/or carbon monoxide. Typically, in a steam reformingprocess, a hydrocarbon-containing feed such as natural gas, steam and anoptional recycle stream such as carbon dioxide, are fed intocatalyst-filled tubes where they undergo a sequence of net endothermicreactions. The catalyst-filled tubes are located in the radiant sectionof the steam methane reformer. Since the reforming reaction isendothermic, heat is supplied to the tubes to support the reactions byburners firing into this radiant section of the steam methane reformer.Fuel for the burners comes from sources such as purge gas from pressureswing adsorption (PSA) unit and some make-up natural gas. The followingreactions take place inside the catalyst packed tubes:

C_(n)H_(m) +nH₂O<=>(n+0.5m)H₂ +nCO

CO+H₂O<=>CO₂+H₂

The crude synthesis gas product (i.e., syngas) from the reformer, whichcontains mainly hydrogen, carbon monoxide, carbon dioxide, and water, isfurther processed in downstream unit operations. An example of steammethane reformer operation is disclosed in Drnevich et al (U.S. Pat. No.7,037,485), and incorporated by reference in its entirety.

Conventional operation of steam reformers limits furnace firing to keepreformer tube wall temperatures at or below the maximum allowableworking temperature (MAWT) for a given process stress, creep-to-rupturetube life target (often 100,000 hours) and safety margin. For example,an HP-Mod tube in a steam methane reformer furnace could have a designtemperature of 1800° F. for 100,000 hours creep-to-rupture targetlifetime and a MAWT of 1750° F., providing a 50° F. safety margin.Optimal firing of a steam reformer strikes a balance between maximizingheat transfer and maximizing tube life. This optimal operating pointoccurs in the idealized scenario when the entire tube surface operatesat the MAWT such that the driving force for heat transfer is large andthe entire tube fails at once after the design creep-to-rupture tubelife target is reached and exceeded.

In reality, tube wall temperatures are not uniform within a reformer,but rather, vary based primarily on the local radiative environment, aswell as on the inside tube heat transfer coefficient, the process gastemperature and composition, the catalyst activity, and the tube thermalconductivity.

In reformers the incident heat flux on a catalyst tube variescircumferentially due to tube-tube shielding, wall-shielding, or otherradiative effects, inducing a circumferential tube wall temperaturegradient. A circumferential tube wall temperature gradient causesnon-optimal tube surface utilization for heat transfer and reduced tubelife. Local radiative environments are primarily a function of thegeometry of the furnace and the respective orientation betweenrelatively hot and cold surfaces. In cylindrical or “can” reformerswhere the tubes are arranged around the circumference of the furnacewith the burner in the center space, the flame-side tube surface canexperience significantly more radiative flux and be significantly hotterthan the side of the tube facing the refractory wall. Similarly, in boxreformers where tubes are arranged in rows with burners firing on eitherside of the tube rows, the flame-side of the tube receives significantlymore radiative flux than the tube side facing either a refractory wallor another tube. Typically, the flame side of the tube surface is hotterthan the tube sides receiving less incident radiative flux. Thistemperature variation is referred to as a “shielding” or “shadowing”effect in the art. Local radiative environments also vary based onelevation within the furnace. For example, the circumferential variationmay be stronger in the top 50% of a down-fired furnace than in thebottom due to the presence of peak flame temperatures at the furnaceinlet. These circumferential tube temperature variations lead to acondition in which some areas of the tube operate with less thermaldriving force for heat transfer. The reformer as a whole is bottleneckedby the hottest tube wall temperatures up to the MAWT, which may only beobserved over a small portion of the tube.

An existing need remains for technologies that can maximize theutilization of the tube heat transfer surface through the elimination ofthe circumferential variations, enabling maximal reformer throughput andfurnace efficiency for a given tube life. Altering the local radiativeenvironment in a given furnace can be capital intensive, potentiallyrequiring physical rearrangement of installed tubes and walls, burnerchanges, or header system reconfigurations, etc. or can be impracticaldue to limitations in flange spacing requirements, etc. Reducing tubetemperatures from the process side (i.e., inside the tube) can beachieved through the utilization of catalysts that promote higher heattransfer or that have higher activity such as structured catalysts orspecially-shaped pellets. Raising/lowering tube temperatures through theadjustment of bulk flow rates through individual tubes is known in theart. Even using differential loadings of catalyst beds with differentpressure drop characteristics to achieve this biasing of flow todifferent tubes in the reformer is known. However, conventionalcatalysts are either randomly packed pellets or structured catalyst withuniform horizontal cross-section, with the intention to distributeprocess flow evenly across the tube cross-section and so do not addressthe problem of circumferential tube temperature variations directly inthe localized way of the present invention.

In the related art, methods to reduce circumferential tube temperaturevariations have primarily been focused on modifying the furnace-to-tuberadiant heat transfer. For instance, some attempts are Krar et al andBuswell et al (U.S. Pat. Nos. 4,098,587 and 4,740,357, respectively)through the use of flue gas radiant shields or through the use ofelliptical tubes rather than circular tubes as shown in Heynderickx andFroment “A Pyrolysis Furnace with Reactor Tubes of Elliptical CrossSection” (1996) Ind. Eng. Chem. Res. 35 pp. 2183-2189 and Sadrameli etal “Shadow Effect Minimization in Thermal Cracking Reactor Coils throughVariable Cross-Section” Scientia Iranica, Vol. 7, No. 2 pp. 137-142.These disclosures rely on controlling the external tube surface heatexchange with the furnace either through manipulation of the externaltube surface exposure to radiant heat transfer or hot flue gases whereasthe current invention deliberately targets controlling the internal tubeheat transfer through the process gas flow pattern.

Several techniques have been brought forward that target increased heattransfer within a steam reformer tube, but do not address thecircumferential tube temperature variation. For example, Whittenbergeret al, Whittenberger et al and Jin et al (U.S. Pat. Nos. 9,216,394;8,721,973, and 8,409,521, respectively) disclose designs for structuredcatalyst that increase the inside tube wall convective heat transfercoefficient by directing process gas into the inside tube wall. Otherrelated art discusses the modification of pellet catalysts to increaseradial mixing and heat transfer through the tube cross section. See,Combs, Birdsall et al, and Cairns et al (International PatentPublication Nos. WO 2004/014549, WO 2010/029323, and WO 2010/029325,respectively). Yet other related art discloses the use of particularpellet catalyst shapes intended to modify the inside tube wall heattransfer coefficient. See, Camy-Peyret et al (International PublicationNo. WO 2014/053553). These designs reduce the maximum tube walltemperature through overall higher heat transfer delivered to theprocess gas and increased reforming. However, these designs do notdeliberately bias process gas toward any particular side of the tubewall. As a result, a circumferential tube temperature gradient stillexists, limiting the operation of the reformer to the hottesttemperature observed on a given side of the tube.

Sato et al (U.S. Pat. No. 4,418,045) and De Angelis et al (U.S. PatentApplication Publication No. 2004/0120871A1) disclose the use ofcatalytic seals (e.g., pellet catalyst, fibrous catalyst, fabriccatalyst, etc.) around the periphery of a structured catalyst bed inorder to prevent flow from bypassing structured catalyst along thereactor wall. However, these seals are intended to prevent bypass flowbetween structured catalyst modules rather than bias flow toward thehigh flux side of the tube wall.

Thus, to overcome the disadvantages of the related art, one of theobjectives of the present invention is to provide a reactor tube with apreferential flow catalyst with a structural element where the processgas flow is directed toward the portion of the tube wall that receiveshigher incident heat flux to reduce the peak tube temperature.

It is another objective of the present invention that thecircumferential tube temperature is reduced by utilizing a catalyst witha structural element that imparts a non-uniform and non-random pressuredrop to the process gas flow, which causes a larger portion of theprocess gas to flow into and react at the portion of the tube wall thatreceives the highest incident heat flux, and a lesser portion of theprocess gas to flow into and react at the side of the tube that receivesrelatively less incident heat flux.

Other objects and aspects of the present invention will become apparentto one skilled in the art upon review of the specification, drawings andclaims appended hereto.

SUMMARY OF THE INVENTION

This invention pertains to a reactor tube packed with a catalyst systememployed to deliberately bias process gas flow toward the tube wall sideof above-average incident flux, and away from the tube wall side ofbelow-average incident flux, in order to reduce the local maximum tubetemperature and, preferentially, to reduce the circumferential tubetemperature variation. In one aspect of the invention, method ofproducing synthesis gas within a tubular reformer is provided. Themethod includes introducing a process gas, where the process gascomprises steam and at least one hydrocarbon at an inlet of one or moretubes disposed in the reformer, contacting the process gas with acatalyst in the interior of the one or more tubes, wherein at least aportion of the catalyst has a structural element that circumferentiallybiases a process gas flow toward at least one tube wall side of greaterincident heat flux thereby reducing the maximum tube wall temperature,and removing a reformed process gas at an outlet of the one or moretubes wherein the reformed process gas is a synthesis gas containingpredominantly hydrogen, carbon monoxide, carbon dioxide, and water.

In another aspect of the invention, catalyst with a structural elementdisposed in the interior of one or more tubes within a tubular reformeris provided. The catalyst includes a structural element that biases alocalized process gas flow, where the process gas stream comprises steamand at least one hydrocarbon, toward the at least one tube wall side ofgreater incident flux.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of the presentinvention will be more apparent from the following drawings, wherein:

FIG. 1(a) is a depiction of a related art reformer with the orientationof the majority of tubes in the interior rows of a box reformer (“innerrow, interior tube”);

FIG. 1(b) illustrates circumferential tube wall temperatures for aninner row, interior tube of a related art box reformer;

FIG. 1(c) is a schematic representation of the orientation of tubesadjacent to a gap and located in an outer row with burners firing atreduced rate in a related art top-fired box reformer (“outer row, gaptube”);

FIG. 1(d) illustrates circumferential tube wall temperatures for anouter row, gap tube of a related art box reformer;

FIG. 1(e) is a schematic representation of the orientation of tubes in arelated art cylindrical—or can—reformer; and

FIG. 1(f) illustrates the ratio of incident local flux density at theoutside tube surface to the maximum tube flux around the circumferenceof a related art cylindrical reformer tube.

FIG. 2 is a plot depicting tube wall temperature for an inner row,interior tube in a box reformer relative to the maximum allowable tubewall temperature.

FIG. 3(a) is a schematic representation of select method to impart abiased flow to process gas across a tube cross-section that has flowresistance elements between catalyst layers;

FIG. 3(b) is a schematic representation of select method to impart abiased flow to process gas across a tube cross-section that has anincreasing fan fold density;

FIG. 3(c) is a schematic representation of select method to impart abiased flow to process gas across a tube cross-section that has flowresistance elements attached to the structure walls;

FIG. 3(d) is a schematic representation of select method to impart abiased flow to process gas across a tube cross-section that has flowresistance elements in the form of thicker structure walls;

FIG. 3(e) illustrates an embodiment, which has a reduction in windownumber and/or size at catalyst periphery in systems where pellets aresupported in structural baskets within tubes; and

FIG. 3(f) illustrates a related art embodiment where an unmodified fanhas a uniform gas flow path area around the structured catalystcircumference.

FIG. 4(a) is a schematic representation of an example CFD simulationwhere there's no modification to catalyst structure (i.e., related art)to impart circumferential flow bias; and

FIG. 4(b) illustrates an embodiment that has flow resistance elementsincluded between catalyst layers to bias flow.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a more efficient heat transfer to theprocess gas in a reformer at a given feed rate and process gas outlettemperature over and above what can be achieved through the related artthat has no process gas flow bias within a tube. For a given elevation,the local convective heat transfer at the inside tube wall surface ismatched to the local incident flux on the adjacent outer tube wallsurface, yielding a lower maximum tube wall temperature for a givenprocess outlet condition and reduced circumferential temperaturegradients. In this manner, a greater portion of the tube surface areaoperates closer to the optimal conditions for maximum heat transfer tothe process gas, fully utilizing the tube heat transfer surface.Circumferential temperature gradients can lead to hoop stresses in thetube materials, but importantly represent non-optimal usage of tube heattransfer surface. In reformer tubes where the catalyst provides no biasto the process gas flow (i.e., the related art), higher tube walltemperatures indicate surfaces with greater incident flux that isunmatched by a suitable uptake of heat on the process side. It is theaim of this invention to use a catalyst system with a structural elementto match the given circumferential variations in the incident flux on anoutside tube wall to a deliberate variation in the circumferentialconvective heat transfer at the inside tube wall, thus reducing the peaktube wall temperature and/or reducing the circumferential tubetemperature gradient.

By way of explanation, FIG. 1 of the related art illustrates thecircumferential variation in the tube wall temperature induced by localradiative and convective heat flux environments for tubes in box and canreformers. As shown in FIG. 1(a), the top view of an inner row tube in atypical box reformer is provided. Each individual tube has two sidesfacing the adjacent interior tubes and two sides facing burner rows.FIG. 1(b) illustrates a computational fluid dynamics (CFD) simulatedcircumferential tube wall temperature profile (i.e., temperatures as afunction of theta (θ)) of reformer tube at approximately one third ofthe way down the fired length of a top-fired box reformer. This tube isadjacent to two similar tubes in the tube row plane and adjacent to twoburner rows firing at similar rates, as shown in FIG. 1(a). The outsidetube wall temperature is highest at θ=0 and π, which are the areasdirectly facing the flames. The circumferential temperature range isgreater than 40° F. with 75% of the tube surface underutilized andoperating at temperatures of more than 10° F. below the flame-sidetemperature, which ultimately limits reformer operation. While thisparticular tube local radiative environment leads to a largely symmetriccircumferential tube temperature profile, this is not a general rule.With reference to FIG. 1(d), a CFD simulated circumferential tube walltemperature profile is depicted (i.e., as a function of theta (θ)) for atube at the same elevation as in FIG. 1(b), but located in an outer rowand next to a gap in the tube row, commonly referred to as a “gap tube”.As illustrated in FIG. 1(c), each side of this tube is adjacent to aunique radiative environment and so the circumferential temperatureprofile is asymmetric.

FIG. 1(f) shows the incident radiative flux along the circumference of atube in a cylindrical or can reformer of the related art. Specifically,the ratio of incident local flux density to the maximum tube flux aroundthe circumference of the outside tube surface of a cylindrical reformertube is depicted. As shown in FIG. 1(e), the tubes in these canreformers have one side facing the flame, one side facing the refractorywall, and two sides facing adjacent tubes. The maximum radiative fluxoccurs at the flame side of the tube.

As shown in FIG. 2, portions of the tube surface that operate below theMAWT are underutilized for heat transfer since, in practice, reformerfiring is limited by the MAWT in order to achieve a desired tube life.In the present invention, the process gas flow is employed to flattenout the observed tube wall temperatures to a temperature at or below theMAWT. In other words, the peaks shown in FIG. 2, are decreased and thevalleys are raised. This achieves a margin between the maximum observedtube temperature and the MAWT, of which operators can take advantage. Insome cases, the peaks can be reduced below the MAWT, giving operatorsroom to take advantage, but the minimum skin temperatures are alsoreduced. This can occur if the effect on two dimensional conduction ofheat through the tube wall from the side of greater incident flux tolesser incident flux is larger than the effect on convective heattransfer at the side of lesser incident flux. For example, this canhappen if a greater portion of flow directed toward the side of greaterincident flux causes a substantially reduced tube temperature, which inturn reduces the driving force for two-dimensional heat conduction tothe side of lesser incident flux, such as in a very thick tube wall.Depending on the balance between the convection and conduction effectsat the side of less incident flux, the minimum tube skin temperature mayraise or lower, but the maximum skin temperature will be reduced,providing net benefit.

The objective of the present invention is to reduce the peak tube walltemperatures to at or below the MAWT and, preferably, to reduce thevariance in the circumferential temperature profile. To achieve this, agreater portion of process gas flow is directed toward the portions oftube wall receiving the highest incident heat flux, and a lesser portionof process gas is directed toward the tube wall sides receiving a lesserheat flux. As utilized herein the terms “high flux side” or “highincident heat flux” of a tube are interchangeable and shall mean theregion of an outside tube wall that receives an above average level ofincident radiative and convective heat flux from the furnace, where theaverage is taken as the circumferential average for that given tube andelevation. In the present invention, a portion of process gas is biasedtoward this side, thereby increasing the process-side local convectiveheat transfer coefficient and lowering the local tube wall temperature.

The “low flux side” or “less incident heat flux” are interchangeable andshall mean the region of an outside tube wall that receives a belowaverage level of incident radiative and convective heat flux from thesurface, where the average is taken as the circumferential average forthat given tube and elevation. A portion of process gas is biased awayfrom this side, thereby lowering the local convective heat transfercoefficient and increasing the local tube wall temperature. This reducesboth the range in circumferential tube wall temperatures and the maximumtube temperature at a given elevation. This flow bias cannot be achievedthrough the use of conventional randomly-packed pellet catalyst orthrough structured catalyst systems that are circumferentially-uniform.These conventional systems are designed to impart a uniform pressuredrop to the process gas flow such that the flow iscircumferentially-even. In order to achieve the flow bias that is theobjective of this invention, it is required to provide a catalyst systemwith an engineered structural element. The structural element can takemany forms, some exemplary embodiments of which are discussed below.

Biasing the process gas flow to the high flux tube wall sectionsincreases the local inside tube wall convective heat transfercoefficient, thereby increasing heat transfer to the local process gas.This increased heat transfer and endothermic reaction will reduce thetube wall temperature at the tube sections with greatest incident flux.Preferably, flow is simultaneously reduced toward tube sections withless incident flux, decreasing the convective heat transfercoefficients, heat transfer, and endothermic reaction locally inside thetube. Together, these will serve to raise the local tube temperature atthe side of the tube receiving less flux and overall balance thecircumferential tube temperature. For the same process gas flow, outletpressure, and process gas exit temperature, the maximum tube walltemperature observed along the tube surface will be reduced. Thisindicates increased furnace efficiency and offers an opportunity toeither bank fuel savings or increase reformer throughput.

The local inside tube convective heat transfer coefficient largelygoverns the rate of heat transfer from the tube wall to the process gas.The local inside tube wall convective heat transfer coefficient isproportional to the Reynolds number raised to power x

h _(tc)˜Re_(θ) ^(x)

where x depends on the mode of heat transfer from the catalyst to thewall. Typical values of x in steam methane reformers range from 0.6 to0.8. The local Reynolds number depends directly on the local velocityvia

${Re}_{\theta} = \frac{\rho VL}{\mu}$

where ρ is the local fluid density, V is the local velocity, L is acharacteristic length, and μ is the local fluid viscosity. In thepresent invention, the structural element of the catalyst can bedesigned such that the radial velocity of gas impinging on the tube wallis proportional to the local radiant and convective flux incident uponthe tube, which can be determined a priori, for example, either throughfurnace observation or calculation with methods such as CFD. For valuesof x between 0.4 and 1, modifications to the catalyst should be madesuch that the ratio V_(high)/V_(low) ranges from 1 to 2.2 where V_(high)and V_(low) are the velocities at the tube wall sides receiving greaterand lesser incident flux, respectively. Using an engineered structuralelement within the catalyst system, the local process gas velocity canbe adjusted to tune the local inside tube heat transfer coefficientaround the circumference of the inside tube wall to match the localincident flux. Such local velocity adjustments cannot be achievedthrough the related art randomly-packed pellet or uniform structuredcatalyst systems.

With reference to FIG. 3, the preferred embodiments of the method andthe catalyst with a structural element are provided for designing asystem to bias flow toward the tube wall sides with highest incidentflux. In some cases, conventional structured catalyst or structuredcatalyst cages for pellets can be used as a basis for the preferentialflow design, but the invention is not limited to only the designs shownin FIG. 3. In many cases, the structure element may be coated with asuitable steam reforming catalyst known in the art, or alternatively thecatalyst itself is structurally designed to bias the flow in accordancewith the invention. FIG. 3a depicts a preferred embodiment utilizingflow resistance elements between catalyst layers to direct flowpreferentially toward the sides of the tube wall with greatest incidentflux. Examples of such flow resistance elements are shown as modifiedwashers separating two catalyst fans in which the flow resistancethrough the washer varies circumferentially. The included examples arein the form of a grate or a perforated plate though other embodimentsare possible. A greater portion of the flow passes through the sides ofthe washer with greater open cross-sectional area and least flowresistance, toward the tube wall with greater incident flux.

FIGS. 3(b), 3(c), and 3(d) depict preferred methods of reducing thecross-sectional area for channels adjacent to tube walls with relativelylower incident radiative flux. This can be accomplished by increasingthe density of channels or folds open to the direction of the portion ofthe tube receiving less incident flux as in FIG. 3(b), by partiallyblocking channels at the periphery of the catalyst as in FIG. 3(c), orby using thicker walls for channels open to the direction of the colderinside tube walls as in FIG. 3(d). Increasing the cross-sectional areaavailable for flow preferentially toward the high flux side of the tubeand increasing the resistance to flow in the direction of the low fluxside of the tube, a greater portion of process gas will tend to flowtoward the high flux tube side. The degree of reduction incross-sectional area needed to adjust the circumferential temperaturesbased incident flux can be determined a priori using either experimentalmethods or calculation (e.g., with CFD tools).

The circumferentially-non-uniform folds such as those shown in FIG. 3(b)are most preferably formed in the initial fan-forming or metalcorrugating process so as to maintain a uniform height of each fan. Auniform height ensures that that the fans can be stacked upon each otherto fill the length of the tube to be filled with catalyst. Thecircumferentially non-uniform folds should be made from a material thatis sufficiently sturdy so as to maintain the folds at high temperatureand through repeated thermal cycles, typical of steam methane reformeroperation.

FIG. 3(c) describes a preferred embodiment of increasing the resistanceto flow toward the sides of the tube with less incident flux via theattachment of flow resistance elements to the peripheral walls of thestructured catalyst. These elements partially impede the flow of processgas through channels leading toward tube walls with less incident flux,thus, allowing a greater portion of the process gas to flow toward thehigh flux tube side with no flow resistance element. In this particularembodiment, the resistance elements are attached to the outside wall ofthe structure, maintaining a gap between the catalyst and the insidetube wall. This allows the elements to be attached in apost-modification process via tack welding or other processes known tothose skilled in the art. However, other embodiments can be envisionedin which the flow resistance elements are attached at the interior wallseither at the initial construction of the structured catalyst or in apost-formation modification process or in which select flow passages arefilled solid.

FIG. 3(e) illustrates a modification to types of reforming catalystsystems of the related art designed to operate with pellet catalyst, butthat enhance heat transfer at the inside tube wall using a structurewhich may or may not be coated with catalyst. The current inventionmodifies such a pellet-structure system to either reduce the relativesizes of holes in the structure open to tube wall sides of lesserincident flux as shown, or reduce the number of holes open to the lowerflux side of the tube. In this embodiment, a greater portion of processgas is deliberately biased toward the higher flux tube wall sides toreduce peak tube wall temperature while preferably reducing flow at thelower flux side simultaneously to reduce the circumferential tubetemperature variation. The sizes of holes in the structure need not beuniform, but can be designed to match the incident heat flux at a localtube wall side such that the peak temperatures are reduced and thecircumferential tube temperature variation at a given elevation isminimized. In the foregoing embodiments the use of a tool like CFD tomatch the catalyst structure adjustments to the local incident flux atthe outside tube wall is preferred.

In all cases, the structural element has an open arrangement that allowsall portions of process gas to be maintained in fluid contact toencourage mixing. It is not preferable to utilize structures or catalystsystems with defined channels that do not allow for the periodicrecombination of process gas portions over the length of the tube.

If the catalytic surface area is reduced too much in the direction ofthe higher flux side wall, the amount of catalyst available to react thenow larger portion of process gas may be too low to achieve the desiredconversion. For example, this can happen if the channels of a structuresuch as that shown in FIG. 3(b) are coated with catalyst, but thecircumferential density of channels is too low to provide the necessarycatalytic surface area. Therefore, the design modification to thestructured catalyst that achieves this reduction in circumferential tubetemperature variation should be balanced with ensuring sufficientcatalytic surface area is available for reaction. One way to compensatefor a reduction in catalytic surface area per unit of process gasvolumetric flow in the direction of the high flux tube wall is to applyadditional coats of catalyst or increase the catalyst loading (e.g., wt% Ni per unit substrate surface area) on these channel walls, therebyincreasing the catalytic surface area.

There are a number of ways to use catalyst with a structural element tobring about the circumferential biasing of process gas flows: In anexemplary embodiment, structured catalyst flow channels are narrowed topreferentially direct flow away from these channels. In anotherexemplary embodiment, flow resistance elements are inserted withinchannels to preferentially direct flow away from these channels. Theseelements may or may not be coated with catalyst. In another exemplaryembodiment, flow resistance elements or baffles are employed betweenstructure channels and the inside tube wall to partially block flowthrough these channels or between the channels themselves. Theseelements may or may not be coated with catalyst. In yet anotherexemplary embodiment, a combination of two or more of these methods isused in concert. In another embodiment, the type and/or cross-section ofthe structural element of the catalyst system is varied along the lengthof the tube and/or from tube to tube based on the local incidentcircumferential heat flux on the tube. Some tubes or some portions oftubes may not incorporate biasing structural elements but rather utilizeconventional catalyst. In yet another exemplary embodiment, the catalystsystems in which pellet catalyst are supported within a structure withflow openings that direct the process gas into the inside tube wall haveeither more holes or larger holes to direct more flow toward the tubewall side receiving higher flux compared with the sides of the structurethat direct flow to the portion of the tube wall receiving less flux.

In a further embodiment, one of these methods for preferentially biasingflow is used and the catalytic activity is increased in the direction ofmore gas flow. This can be accomplished, for example, by using higheractive metal loadings in the catalytic coatings of structured catalystor by using pellet catalyst with higher catalytic surface area. Thecatalyst is designed to induce a circumferential bias in process gasflow and may fill the entire tube length or may be utilized in only aportion of the tube (e.g., where tube wall temperatures are highest),wherein up to the remainder of the tube is filled with conventionalunbiased structured catalyst, pellet catalyst, or a combination thereof.

In a further embodiment, the catalyst is designed to induce acircumferential bias in process gas flow can be installed in all or inonly particular types of tubes (e.g., end tubes, corner tubes). Ifneeded, a mechanical mechanism is used to fix the structured catalystinto place and prevent rotation during operation.

The invention is further explained through the following example, whichcompares the base case with a standard design at the outlet tube, andthose based on various embodiments of the invention, which are not to beconstrued as limiting the present invention.

Comparative Example

This example illustrates how the insertion of flow resistance elementsbetween two catalyst layers can be used to bias flow toward the tubewall with greater incident flux and achieve a reduction in both thespread of the circumferential tube temperatures as well as the maximumtube temperature. Computational Fluid Dynamics (CFD) was used tosimulate the non-uniform heat flux around a reformer tube wall such asmight be found in an up-fired cylindrical reformer tube. Boundarycondition heat fluxes of 12000 W·m⁻² and 10000 W·m⁻² were each appliedto one half of the tube metal skin as shown in FIG. 4. This conditionmimics the case for one side of the tube wall facing the flame withhigher incident flux and one side facing other tubes or the furnacerefractory wall with less incident flux. Cases without circumferentialflow bias (FIG. 4(a)) and with modifications to impart circumferentialflow bias (FIG. 4(b)) through the addition of inter-layer flowresistances were simulated to show impact. A consistent total inlet flowrate was applied in both cases. In order to bias flow preferentiallytoward the side with highest incident flux, flow resistances wereapplied between catalyst layers such that 60% of the incoming flow, Q,was directed toward the tube side with higher incident flux and 40% wasdirected toward the side with less incident flux, as shown in FIG. 4(b).These flow resistances can be achieved via many methods, two of whichare shown in FIG. 3(a).

As shown in Table 1, below, using flow resistance elements to bias flowtoward the side of the tube with greater incident flux reduces themaximum tube temperature by 23° F. and reduces the variation in the tubeskin temperatures from 58° F. to 35° F. The MAWT for this simulated tubeis set at 1775° F. In the case with no flow bias, as in FIG. 4(a), thereis no margin between the maximum tube temperature and the MAWT, limitingfurnace efficiency and throughput. However, using the catalyst with astructural element that imparts a flow bias achieves a 28° F. marginbetween the MAWT and the maximum tube temperature, which allows thereformer operator to, for example, increase throughput or take advantageof increased tube life.

TABLE 1 Flow resistance No between Case modification catalyst layersmaximum tube skin temperature (° F.) 1775 1747 minimum tube skintemperature (° F.) 1718 1712 skin temperature spread (° F.) 58 35average skin temperature (° F.) 1746 1723

CFD simulation of reduction of circumferential tube skin temperaturevariation and maximum skin temperature using flow resistance elementsbetween catalyst layers to bias flow toward tube wall with greatestincident flux.

Although various embodiments have been shown and described, the presentdisclosure is not so limited and will be understood to include all suchmodifications and variations as would be apparent to one skilled in theart.

1-6. (canceled)
 7. A catalyst with a structural element disposed in theinterior of one or more tubes within a tubular reformer, comprising: astructural element that circumferentially biases a process gas flow,where the process gas stream comprise steam and at least onehydrocarbon, toward the at least one tube wall side of greater incidentheat flux.
 8. The structural element of claim 7, wherein the structuralelement directs flow away from tube wall sides of lesser incident heatflux.
 9. The structural element of claim 7, wherein the structuralelement is a flow resistance element disposed between catalyst sectionsalong the length of the tube.
 10. The structural element of claim 9,wherein the flow resistance elements are perforated plates or grateswith circumferentially non-uniform open channels.
 11. The structuralelement of claim 9, wherein the flow resistance elements have a lesserflow resistance toward the sides of the tube with greater incident flux,thereby biasing flow toward these tube wall sides.
 12. The structuralelement of claim 11, wherein the flow resistance elements have a greaterflow resistance toward the sides of the tube with lesser incident heatflux, thereby biasing flow away from these tube wall sides.
 13. Thecatalyst with a structural element of claim 7, wherein the catalyst isin pelletized form supported by the structural element where saidelement is a perforated metal basket having non-uniform flow openingsthat preferentially directs flow.
 14. The structural element of claim13, wherein a greater portion of flow opening is disposed adjacent thetube wall sides with greater incident heat flux.
 15. The structuralelement of claim 13, wherein a lesser portion of flow opening isdisposed adjacent the tube wall sides with less incident heat flux. 16.The catalyst of claim 7, wherein the structural element is coated withsaid catalyst.
 17. The structural element of claim 7, wherein thestructural element is a flow resistance element that is selected fromthe group comprising of fan folds, thickened baffles, and structuralbaskets.
 18. The catalyst with a structural element of claim 7, whereinthe catalytic activity is increased in the direction of biased gas flowby employing a catalyst with higher active metal loading on the tubesides with greater incident heat flux than on the tube sides with lesserincident heat flux.
 19. The catalyst with a structural element of claim13, wherein the catalyst activity is increased in the direction ofbiased gas flow by employing a pellet catalyst with higher catalyticsurface area on the tube sides with greater incident heat flux than onthe tube sides with lesser incident heat flux.